Natural gas is produced from various geological formations. Natural gas produced from geological formations typically contains methane, ethane, propane, and heavier hydrocarbons, as well as trace amounts of various other gases such as nitrogen, carbon dioxide, and hydrogen sulfide. The various proportions of methane, ethane, propane, and the heavier hydrocarbons may vary, for example, depending upon the geological formation from which the natural gas is produced.
For example, natural gas produced from conventional geological formations, such as reservoir rock formations, may comprise about 70-90% methane and about 3-9% ethane, with the remainder being propane, heavier hydrocarbons, and trace amounts of various other gases (nitrogen, carbon dioxide, and hydrogen sulfide). Such conventionally-produced natural gases may be termed “lean,” meaning that this natural gas contains from about 2 to about 4 gallons of ethane per thousand standard cubic feet of gas (GPM).
Conversely, natural gas from unconventional geological formations, such as coal seams, geo-pressurized aquifers, and shale formations, may comprise about 70-80% methane and about 10-25% ethane, with the remainder being propane, heavier hydrocarbons, and trace amounts of various other gases (nitrogen, carbon dioxide, and hydrogen sulfide). Such non-conventionally-produced natural gases may be termed “rich,” having 8-12 GPM.
During natural gas processing, the natural gas produced from a geological formation (e.g., the “feed gas”) is generally separated into two product streams: a natural gas liquids (NGL) stream and a residue gas stream. In some circumstances, it may be desirable that the ethane within the feed gas stream is separated into the resulting NGL stream (referred to as an “ethane recovery” configuration). Alternatively, it may be desirable that the ethane within the feed gas is separated into the resulting residue gas stream (referred to as an “ethane rejection” configuration).
Conventional natural gas separation systems and methods are generally designed and built to be operated so as to recover ethane as a component of the NGL stream. As such, operating a conventional natural gas processing system or method such that ethane is rejected, that is, so that ethane is present in the residue gas stream, is outside the design parameters upon which such conventional systems and methods are based, resulting in decreases in operational efficiency.
Further, conventional natural gas separation systems and methods are also generally designed and built to be operated within relatively narrow ranges of parameters, for example, as to feed gas composition and throughput rate. Operating such a conventional natural gas processing system or method outside of these parameters (for example, by processing natural gases having a composition other than the range of composition for which the system/method was designed and built and/or processing natural gas at a throughput rate other than the rate for which the system/method was designed and built) may be so inefficient as to be economically undesirable or, may be impossible because of system limitations.
As such, what is needed are cost effective systems and methods for processing natural gas (i) that may be used to selectively recover or reject ethane, (ii) that may be used to process natural gas having variable composition (e.g., natural gas from conventional or non-conventional geological formations), and (iii) that may be used to process natural gas at a wide range of throughput flow-rates, while achieving high propane recovery, particularly during ethane rejection.